What is Electron Configuration?
Electron configuration is the distribution of electrons of an atom or molecule in the atomic or molecular orbitals. Commonly, the electron configuration is used to describe the orbitals of an atom in its ground state, but it can also be used to represent an atom that has ionized into a cation or anion by compensating for the loss of or gain of electrons in their subsequent orbitals. Writing down the electronic configuration is a way of finding the effective valency of the element or ion.
How To Write Electronic Configuration?
Basically, the distribution of electrons over various shells (energy levels) is such that, the overall energy of all electrons of an atom remains minimum for the stability of the atom. The electron distribution over various energy level is governed by the following rules:
- Electrons fill orbitals in a way to minimize the energy of the atom. Therefore, the electrons in an atom fill the principal energy levels in order of increasing energy (the electrons are getting farther from the nucleus). The order of levels filled looks like this:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p
To help memorize this sequence check out this arrangement,
Pauli Exclusion Principle:
According to this principle, an orbital can have maximum two electrons and these must have opposite spins. In other words, no two electrons in an atom can have all the four quantum numbers exactly the same. As we know that fourth quantum number is m, 1/2 is spin up and -1/2 is spin down and hence if the first three quantum numbers are same for two electrons then the electrons must have opposite spins in an orbital.
When assigning electrons in orbitals, each electron will first fill all the orbitals with similar energy (also referred to as degenerate) before pairing with another electron in a half-filled orbital.
According to this principle, the electrons will first occupy the orbitals that have the lowest energy.
- Although the Aufbau rule accurately predicts the electron configuration of most elements, there are notable exceptions among the transition metals and heavier elements. The reason these exceptions occur is that some elements are more stable with fewer electrons in some subshells and more electrons in others.
- When writing an electron configuration, first write the energy level (the period), then the subshell to be filled and the superscript, which is the number of electrons in that subshell. The total number of electrons is the atomic number, Z. The rules above allow one to write the electron configurations for all the elements in the periodic table.
For example, the Electronic configuration of iron can be written as:
1s2 2s2 2p6 3s2 3p6 3d6 4s2 = [Ar] 3d64s2
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